Molecular tests of RNA sequence heterogeneity at positions of type 3 poliovirus genome at which attenuated Sabin strain differs from its wild- type progenitor Leon/37 showed that positions 472 and 2493 contain the mixture of U and C in various vaccine preparations. Monkey tests performed with virus preparations containing different amount of mutant virus at both positions showed that C at position 472 strictly correlates with the neurovirulence in monkeys, while the abundance of revertants at position 2493 fails to correlate with this property. Passaging of the virus in different cell cultures results in a rapid selection of revertants at positions 472 and 2493. Different factors including the temperature of growth, multiplicity of infection, seed virus, cell type and cell culture density were shown to affect the rate of mutant selection. One paper was published in Proc. Natl. Acad. Sci. USA 88: 199-203 (1991), and recently a manuscript was submitted to the Journal of Virology. Results were presented at the UCLA Symposium on "Molecular Biology of Human Pathogenic Viruses", Lake Tahoe, March 8-15, 1991 and at the WHO Informal Consultations on "New Approaches to Poliovirus Diagnosis Using Laboratory Techniques", Geneva, June 6-7, 1991. A poster will be presented at the 1991 ASCB/EMBV Summer Conference (Switzerland, August 1991) and the abstract published.